On the Relative Surface Density Change of Thermally Unstable Accretion Disks

The relations among the relative changes of surface density, temperature, disk height, and vertically integrated pressure in three kinds of thermally unstable accretion disks are quantitatively investigated, assuming local perturbations. The surface density change is found to be very small in the long perturbation wavelength case but cannot be ignored in the short-wavelength case. It becomes signiÐcant in an optically thin, radiative coolingÈdominated disk when the perturbation wavelength is shorter than 15H (where H is the scale height of disk) and in a geometrically thin, optically thick and radiation pressureÈ dominated disk when the perturbation wavelength is shorter than 50H. In an optically thick, advectiondominated disk, which is thermally unstable against short-wavelength perturbations, the relative surface density change is much larger. We prove the positive correlation between the changes of surface density and temperature in an optically thick, advection-dominated disk that has previously been claimed to be the essential point of its thermal instability. Moreover, we Ðnd an anticorrelation between the changes of disk height and temperature in an optically thick, advection-dominated disk. This is the natural result of the absence of appreciable vertically integrated pressure change. Subject headings : accretion, accretion disks È black hole physics È instabilities